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Technical Why do they call them hemis Dad?

Discussion in 'The Hokey Ass Message Board' started by foolthrottle, Mar 22, 2015.

  1. foolthrottle
    Joined: Oct 14, 2005
    Posts: 1,272

    foolthrottle
    Member

    Years ago I had one of my contraptions with a hemi at a car show, and a lot of people would say things like, is that a 426? ( it was a 354) or what kind of engine is that? etc. then this guy walks up with his 12 year old and asked "why do they call them hemis Dad? the Dad then begins to describe the half hemisphere (rounded) combustion camber, with spark plug in the center (no blind spots) and an intake valve at the top of the combustion chamber and an exhaust valve at the bottom, because of the way the head flowed a lot of early drag racers used them with super chargers. Anyone else got a description?
     
  2. aaggie
    Joined: Nov 21, 2009
    Posts: 2,531

    aaggie
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    Hemi = hemispherical shape of the combustion chamber, period.
     
  3. Hnstray
    Joined: Aug 23, 2009
    Posts: 12,307

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    In my opinion, the valves are not at the top and bottom of the combustion chamber....I believe the reference angle is the verticle line of the cylinder bore, regardless of the incline of the cylinder itself.

    Other than that minor quibble, I think the guy was pretty much on target. It could be elaborated.

    Ray
     
    Last edited: Mar 22, 2015
  4. foolthrottle
    Joined: Oct 14, 2005
    Posts: 1,272

    foolthrottle
    Member

    The reason to bother was simply information, someone like yourself who could probably assemble one in your sleep wouldn't get anything out of it, but my guess is others already have.
     

  5. Hemi is Greek for the Latin semi which means half. Hemispherical meaning half of a sphere. The numbers just reference the total displacement of the cylinders in the engine.

    The shape lends itself to a very efficient Venturi effect with little to no detriment to flow.

    Also, the shape works to remove any sharp edges which will create hotspots and allows for a measurably higher compression ratio with a given type of fuel.

    It's physics, but take it with a grain of salt! ;)
     
    prewarcars4me likes this.
  6. foolthrottle
    Joined: Oct 14, 2005
    Posts: 1,272

    foolthrottle
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    really good
     
  7. Hnstray
    Joined: Aug 23, 2009
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    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    Ok.....so the greater surface area of the hemispherical chamber allows for larger valves, cross flow placement of the ports, no shrouded areas...they simply 'breathe' better than other configurations, central location of the spark plug promotes more even and complete burning of the fuel charge......and.....the head configuration/rocker covers look impressive as well. What's not to like? :)

    Ray
     
    Last edited: Mar 23, 2015
  8. Larry T
    Joined: Nov 24, 2004
    Posts: 7,740

    Larry T
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    The kids dad nailed the definition.

    BTW, Chrysler didn't invent the design. Harley started using it in 1936 on their first knuckleheads and I'm sure they weren't the first.

    There can be some shrouding in the combustion chamber (piston dome). Pretty sure that's why there are some dual plug heads (Chrysler and Harley).
     
    Last edited: Mar 23, 2015
  9. LM14
    Joined: Dec 18, 2009
    Posts: 1,937

    LM14
    Member Emeritus
    from Iowa

    There were Hemi engines at the turn of the last century, either 1901 or close to that. Several early European cars had hemi engines.

    SPark
     
  10. foolthrottle
    Joined: Oct 14, 2005
    Posts: 1,272

    foolthrottle
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    Come on Ray, I know you got more. My understanding was the design originated for aircraft some time in the thirties, I ran a couple of mine in Mexico at high altitude and although they suffered from it, not quite as much as other engines. I use the 55 heads that came with hardened valve seats from the factory. My engines seemed to like the low octane Pemex and plenty of it. Chrysler did make an experimental hemi inline 6 that never made production, Chrysler Australia made one that did.
     
  11. The37Kid
    Joined: Apr 30, 2004
    Posts: 28,856

    The37Kid
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    The 1902 Truscott was featured on The Old Motor website a while back, it had an overhead camshaft. Bob
     
  12. ...
     
    Last edited: Sep 24, 2016
  13. The hemispherical description of the dad is probably as close as you are going to get to a good description.

    Here is something interesting that someone said a while back, the 426 is not considered to be a "real" hemi because the combustion chambers are not deep enough to be half a sphere. I didn't say it and I am not sure that I totally buy into it but I guess from a purists standpoint I understand what they were saying.

    Just a side bar there.

    I think that to be a hemi aside from being a co ross flow head the valves need to be symmetrically placed across from each other. But that would be a lot for a young fella to take in not seeing the valve layout of two heads for comparison.
     
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  14. donno
    Joined: Feb 28, 2015
    Posts: 426

    donno
    ALLIANCE MEMBER

    someone mentioned the Hemi being used in an aircraft, this is true. Republic Aircraft used 2 Hemis, in tandem, inverted ( provided better vision) in a modified P-47 ( X-P-47-N) Can't remember all the details, but the advent of Turbine's slowed any more development. I believe 2 Aircraft were completed and test flown. I don't know how many engines were built, biut heard one is in a Chrysler Museum in Detroit.
     
  15. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,614

    Rusty O'Toole
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    Hemi head engines were built as far back as 1904. In 1912 the French Peugeot company made a hemi head racing engine that was very successful, and was copied all around the world. Practically every purpose build racing engine since has been a hemi.

    Some expensive European cars like Bentley took up the hemi design right away, but as a rule it was considered too expensive, too complicated and too noisy for everyday use.

    Stutz and Duesenberg made hemi head straight eights in the thirties but it was Chrysler who made the first mainstream, mass produced hemi in America.

    Today all kinds of 4 cylinder and V6 hemis are made. Since the 1970s pollution laws and low octane gas gave the open chamber hemi head the advantage in power, mileage and pollution control and many companies have taken it up especially in Japan.
     
  16. Yep the Chrysler hemi was originally intended to be used as an air craft engine. From what I have read one of the things that slowed production was valve wear and that eventually led to hardened seats for the production motors.
     
  17. The37Kid
    Joined: Apr 30, 2004
    Posts: 28,856

    The37Kid
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    Is this a "Bristol" design of rockers on a HEMI? ARDUN used a setup like this on the V8-60 heads the save on the long rocker on the exhaust valve. Bob
     
  18. foolthrottle
    Joined: Oct 14, 2005
    Posts: 1,272

    foolthrottle
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    I don't know of a v8 hemi as an aircraft engine. What I meant was the design of the combustion chamber was for aircraft.
     
  19. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,614

    Rusty O'Toole
    Member

    Bristol was a copy of the prewar BMW engine. The BMW hemi head was an adaptation on an earlier OHV block.

    The Ardun and Chrysler pushrod and rocker arm layout do resemble each other. But Chrysler had their first experimental car engine running before the Ardun was on the market.

    Chrysler also built a 2000 HP V12 aircraft engine with hemi heads during WW2. It was never produced because about the time they finished development work the jet engine made it obsolete.
     
    Last edited: Mar 24, 2015
  20. Dick Stevens
    Joined: Aug 7, 2012
    Posts: 3,344

    Dick Stevens
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    The kid's Dad did an excellent job of explaining it to his 12 year old son, I'll bet they build hot rods together! ;)
     
  21. The37Kid
    Joined: Apr 30, 2004
    Posts: 28,856

    The37Kid
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    Not V8 hemis but Bugatti used two Twin Cam straight eights to power his Air Speed Record chalenger. Bob
     
  22. fridaynitedrags
    Joined: Apr 17, 2009
    Posts: 402

    fridaynitedrags
    Member

    Two things I have been told over the years.......
    1. The hemi combustion chamber was all about flame propogation.
    2. The late 426 hemi was not a true hemi, it was offset by 10 degrees in order to make it fit in the available engine bays of the mid-sixties cars.
     
  23. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,614

    Rusty O'Toole
    Member

    1) Flame propagation

    2) Room for bigger valves

    3) Room around the valves for ample cooling passages (especially important on exhaust valves)

    4) Ports have a straight shot at the valves, no tortured port shapes

    5) Wide open combustion chamber means air can flow in and out easily.

    6) Compact head shape means less heat energy absorbed into the head.
     

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